Many people don't realize that a compass needle that is free to move up and down actually dips into the Earth, with the dip depending on latitude. To counteract this, in most compasses the needle rests on a pin at a position slightly off center. If you buy a compass intended for use in the Northern Hemisphere, and then take it to the Southern Hemisphere, it will no longer be level, making it much harder to use.

Interesting. I had the right physical principle, but the wrong implementation in mind. I would have suggested finding north with the compass and then turning it on its side to try to determine inclination. I didn't realize you had to weight the compass to compensate. I should have thought about it more.